The present invention relates to ring furnaces and more particularly to an improved baffle in the side wall flues thereof for producing improved, for example, carbonaceous anodes for use in producing aluminum metal by electrolysis of alumina dissolved in a molten salt electrolyte.
As explained by J. Z. Nelson at the AIME Annual Meeting, Washington, D.C., Feb. 18, 1969,
A carbon baking ring furnace is made up of a number of pits for the anodes, arranged in rows or "sections" in two halves of a suitable building. The number of pits per section ranges from six to nine, and the number of sections per furnace varies from 60 to 96. Pit size is chosen to suit the anodes to be baked -- pits are about three feet wide, nine to ten feet long, and nine to ten feet deep. Refractory flues make up the side walls of each pit, and refractory headwalls form the ends of the pits. The flues are connected in series. Firing and waste gas equipment is movable, so that four to six fires can move around the furnace in procession.
Anodes are packed in the furnace pits in layers, either upright or on end, with sized coke packing material for support as the anodes soften during baking; and with a top blanket of coke to insulate and seal the pit. The packing coke may contain volatiles -- these and volatiles from the anode binder are burned inside the flues (operating at reduced pressure), contributing to fuel input. Cathode blocks may also be baked in ring furnace pits, and cokes and anthracite coals may be calcined.
From 13 to 18 sections in a series are needed for the operation of one "fire" in a ring furnace. A "fire" is a series of burners, arranged with one to each flue in a section. Considering one such fire in a ring furnace, the sections involved would be:
A. one to three sections will have cooled, and are being unpacked. Pit temperatures will be 200.degree. - 300.degree.C. PA1 B. five to seven sections of baked anodes will be cooling. Combustion air for the fire will be drawn through the flues of some or all of these sections. depending on draft capacity. Pit temperatures will range from 400.degree. to 1150.degree.C. Air to the baking section will be 800.degree. - 1100.degree.C. PA1 C. one section will be baking. During the firing time of 40 to 60 hours, pit center temperatures in that section will be raised from 800.degree. - 900.degree.C. to 1100.degree. - 1200.degree.C. PA1 D. two to four sections of green anodes will be preheated by the waste gas passing through their flues. Pit temperatures thus range from "cold" up to 800.degree. - 900.degree.C.; while the waste gas cools from 1300.degree. - 1400.degree.C. down to 300.degree. - 400.degree.C. PA1 E. one to three sections of green anodes will be waiting their turn at preheating as the fire moves toward them. PA1 F. one to three sections of pits will be empty, for any refractory maintenance and for reloading.
For extensive, detailed illustrations of the design of ring furnaces, reference is made to U.S. Pat. Nos. 1,330,164, 1,330,175, and 1,351,281.
These patents have their individual idiosyncrasies, but they all operate essentially as described by J. Z. Nelson.
With respect to 1,330,164, a peculiarity of that design is that the flame first extends out under a cover and over the pits. The present inventors prefer to completely insulate the tops of the pits and to fire directly into the flues of the pit side walls. Also, it is the preferred practice of the present inventors not to run the air or combustion gases underneath the pits, but rather to let them pass directly from side wall flue to side wall flue.
In 1,330,175, it will be seen that the art had already begun to abandon the technique of initially firing underneath a cover and over the pits. Thus, in 1,330,175, firing is directly into the flues in the pit side walls. However, here, too, there is a difference between what was done earlier and the presently preferred practice of the inventors, in that the firing is effected with the flame direction horizontal into the upper part of the flues. In contrast, presently preferred practice is to direct the fuel downwardly into the flue being fired.
In 1,351,281, we have an example of the art's using the downwardly directed introduction of the fuel into the flue in the manner preferred by the inventors.
In the TMS PAPER SELECTION Paper No. A69-26, of the Metallurgical Society of AIME, entitled "Operation of Ring Type Anode Baking Furnaces - Methods of Improving Baked Anode Quality" by R. C. Abrahamson, W. F. Barrier, and A. O. Pinner, which was presented at the TMS-AIME Annual Meeting, February 17-20, 1969, at Washington, D.C., it is explained that a desired goal in the operation of ring furnaces is the maintaining of the temperature within the pits as uniform as possible.